I'm 23 and I start training in a couple of weeks time. The average age of people at my PRTC was actually 21 rather than 16-18. I don't think it really matters how old you are, the fact you are a graduate and may have more life experience would make me suggest you apply for an Officer/SNCO role. However if you are set on the roles you have mentioned as an airman, then go for it. I'd pop down to your local AFCO and explain you are a graduate, working in a supermarket and are interested in the roles above. You'll get an idea of each role and all the information you need to make a decision to apply.